The characteristics whereby nematic or nematic-cholesteric liquid-crystalline materials significantly vary their optical properties, such as light absorption, light scattering, birefringence, reflectivity or color, under the influence of electric fields, are widely utilized for electro-optical display elements. The functioning of display elements of this type is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the technical application of these effects in electronic components, liquid-crystalline dielectrics are required which must meet a large number of demands. Chemical resistance to moisture, air and physical influences, such as heat, infrared, visible and ultraviolet radiation, and continuous and alternating electric fields, is of particular importance. Industrially usable liquid-crystalline dielectrics are also required to have a liquid-crystalline mesophase in the temperature range from at least +10.degree. C. to +50.degree. C., preferably from 0.degree. C. to 60.degree. C., and the lowest possible viscosity at room temperature, which preferably should not exceed 70.times.10.sup.-3 Pa.s. Finally, they must not have any characteristic absorption in the visible region, that is, they must be colorless.
A number of liquid-crystalline compounds has already been disclosed, which fulfill the stability demands made on dielectrics intended for electronic components, and which are also colorless. These include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628 and the p,p'-disubstituted phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684. In both these classes of compounds, and also in other known series of compounds with a liquid-crystalline mesophase, there are no individual compounds which form a liquid-crystalline nematic mesophase in the required temperature range from 10.degree. C. to 60.degree. C. As a rule, mixtures of two or more compounds are therefore prepared in order to obtain substances which can be used as liquid-crystalline dielectrics. For this purpose, at least one compound having a low melting point and clear point is usually mixed with another compound having a markedly higher melting point and clear point. This normally gives a mixture, the melting point of which is below that of the lower-melting component, while the clear point is between the clear points of the components. Nevertheless, difficulties arise again and again in the preparation of optimum dielectrics, because the components having the high melting points and clear points frequently also impart a high viscosity to the mixtures. As a result, the switching times of the electro-optical display. elements produced with these mixtures, are extended in an undesirable manner. Moreover, problems are frequently caused by the fact that the mutual solubility of the various components, in particular at room temperature or lower temperatures, is only very limited.